nanomaterials Article Photodehydrogenation of Ethanol over Cu2O/TiO2 Heterostructures Congcong Xing 1,2, Yu Zhang 1, Yongpeng Liu 3 , Xiang Wang 1, Junshan Li 1, Paulina R. Martínez-Alanis 4 , Maria Chiara Spadaro 5 , Pablo Guardia 1, Jordi Arbiol 5,6 , Jordi Llorca 2,* and Andreu Cabot 1,6,* 1 Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Spain; [email protected] (C.X.); [email protected] (Y.Z.); [email protected] (X.W.); [email protected] (J.L.); [email protected] (P.G.) 2 Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain 3 Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO), École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland; yongpeng.liu@epfl.ch 4 ENFOCAT-IN2UB, Universitat de Barcelona (UB), C/Martí i Franquès 1, 08028 Barcelona, Spain; [email protected] 5 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; [email protected] (M.C.S.); [email protected] (J.A.) 6 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain * Correspondence: [email protected] (J.L.); [email protected] (A.C.) Abstract: The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process requires the use of highly active, stable and selective photocatalytic materials based on abundant elements and the proper adjustment of the reaction conditions, including temperature. In this Citation: Xing, C.; Zhang, Y.; Liu, Y.; work, Cu2O-TiO2 type-II heterojunctions with different Cu2O amounts are obtained by a one-pot Wang, X.; Li, J.; Martínez-Alanis, P.R.; hydrothermal method. The structural and chemical properties of the produced materials and their Spadaro, M.C.; Guardia, P.; Arbiol, J.; activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. Llorca, J.; et al. The Cu O-TiO photocatalysts exhibit a high selectivity toward acetaldehyde production and up to Photodehydrogenation of Ethanol 2 2 tenfold higher hydrogen evolution rates compared to bare TiO . We further discern here the influence over Cu2O/TiO2 Heterostructures. 2 Nanomaterials 2021, 11, 1399. https:// of temperature and visible light absorption on the photocatalytic performance. Our results point doi.org/10.3390/nano11061399 toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy for solar energy conversion. Academic Editor: Giuseppe Marcì Keywords: titanium dioxide; copper oxide; photodehydrogenation; ethanol; thermo-photocatalysis; Received: 28 April 2021 hydrogen Accepted: 21 May 2021 Published: 25 May 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Molecular hydrogen, a clean energy carrier and a key component in the chemical published maps and institutional affil- industry, is mostly produced through partial oxidation and steam reforming of natural iations. gas and coal gasification. To move away from the exploitation of fossil fuels, cost- and energy-effective strategies for the direct production of hydrogen from renewable sources need to be defined. In this context, biomass resources are a particularly compelling alterna- tive source of hydrogen owing to their renewable character and their near net-zero CO2 Copyright: © 2021 by the authors. footprint [1–6]. Additional advantages of the hydrogen production from dehydrogenation Licensee MDPI, Basel, Switzerland. of biomass-derived organics are the potential to co-produce valuable side organic chem- This article is an open access article icals for better process economics and the possibility to implement cost-effective waste distributed under the terms and abatement processes [7,8]. conditions of the Creative Commons Among the possible dehydrogenation processes, photocatalytic routes that make use of Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ ubiquitous, abundant and renewable solar energy are especially attractive. Photocatalytic 4.0/). processes also enable the dehydrogenation reaction to take place in milder conditions, Nanomaterials 2021, 11, 1399. https://doi.org/10.3390/nano11061399 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1399 2 of 15 which further decreases costs and can increase the side product selectivity compared with thermocatalytic analogs [2]. From another point of view, the photocatalytic production of fuels can be considered as a convenient strategy to store intermittent solar energy [9,10]. In this scenario, the photodehydrogenation of ethanol to produce molecular hydrogen and acetaldehyde using solar light as the only energy input is especially appealing [6,11]. As a liquid, ethanol can be easily stored and transported. Besides, ethanol can be easily produced from several biomass-derived feedstocks and organic residues such as sewage sludge [12–14]. Additionally, bioethanol aqueous solutions can be directly used, without the need for purification. Compared with water splitting, the production of hydrogen from ethanol is thermodynamically advantageous (DG0 = +237 kJ·mol−1 for water oxidation vs. DG0 = +41.5 kJ·mol−1 for ethanol oxidation to acetaldehyde), which decreases the energy input required to drive hydrogen production [2,8]. Compared with water splitting, ethanol dehydrogenation also enables a much simpler product purification, preventing the H2 and O2 back reaction. Besides, compared with ethanol photoreforming, ethanol photodehydrogenation to H2 and acetaldehyde could have a threefold higher economical profitability associated with the high economic value of the side product [15]. In terms of catalysts, while photocatalytic water splitting requires semiconductors with conduction and valence band edges sufficiently above and below the potentials for H+ reduction and water oxidation, respectively, ethanol dehydrogenation can be activated in semiconductors with significantly lower band gaps. On the other hand, the catalytic dehydrogenation of ethanol competes with the deoxygenation, reforming and decomposi- tion reactions, which makes the selectivity of the catalytic process fundamental to ensure cost-effectiveness [1]. Copper oxides, Cu2−xO, have raised increasing attention as photocatalytic materials owing to their abundance, low cost, minor environmental and health impact and suitable optoelectronic properties. Cu2−xO are p-type semiconductors with a very energetic con- duction band and a relatively low bandgap: 2.1 eV for Cu2O and 1.2 eV for CuO, which enables absorption of the visible range of the solar spectra. As a drawback, Cu2−xO have poor photostability, being prone to photocorrosion in reaction conditions. Besides, Cu2−xO generally presents a large defect density that results in a relatively fast recombination of photogenerated charge carriers. To solve these limitations, Cu2−xO can be combined with TiO2 within p-n heterojunctions that protect Cu2−xO against photocorrosion and reduce the charge carrier recombination. The synergism between the two materials is enabled by the appropriate conduction band edges of Cu2−xO, −1.79 V for Cu2O and −1.03 V for CuO, which allows the rapid injection of the photogenerated electrons from the Cu2−xO to the TiO2 conduction band [8,15–19]. Thus, the combination of Cu2−xO and TiO2 is regarded as a highly interesting photocatalyst to: (i) stabilize the Cu2−xO, (ii) boost the overall catalytic activity by extending the light absorption of TiO2 toward the visible light range and (iii) maximize external quantum yield by a rapid charge separation between the two phases enabled by their adequate band edges. While the concept of a p-n heterojunction between Cu2−xO and TiO2 that promotes catalytic activity is pleasantly simple, real systems are much more complex, and Cu2−xO have been reported to promote catalytic activity through several different mechanisms: (i) Cu2−xO can absorb the visible light and transfer photogenerated electrons to TiO2, where H2 evolves, while using photogenerated holes to oxidize the organic species [20]. (ii) Cu2−xO can absorb visible light but use photogenerated electrons to evolve H2 and recombine in photogenerated holes at the Cu2−xO/TiO2 interphase within a Z-scheme mechanism [21]. (iii) Cu2−xO nanoparticles can be reduced to metallic copper during ethanol photodehydrogenation, and the resulting metal nanoparticles can act as a co- catalyst, stabilizing photogenerated electrons, promoting the water reduction reaction, simultaneously reducing the rate of charge recombination and, thus, also making more holes available for the oxidation reaction [15,22,23]. (iv) Cuδ+ and Cu0 on the surface of supported Cu clusters can also participate as catalysts in the ethanol oxidation to acetalde- hyde [17]. (v) Copper ions can be partially incorporated into the TiO2 lattice by substituting Nanomaterials 2021, 11, 1399 3 of 15 4+ for Ti ions and creating oxygen vacancies that decrease the TiO2 bandgap [15,19,24,25]. All these effects strongly depend on the synthesis procedure, the TiO2 surface area and its structural and chemical properties, which affect the Cu dispersion and oxidation states [22] and the TiO2 phase that also determines the interaction with Cu and the Cu role [26]. Most previous works assign the